1. Field of the Invention:
The present invention relates to an ultrasonic motor which is capable of obtaining a high torque by stably generating vibrations with a large vibrational amplitude.
2. Description of the Related Art:
Ultrasonic motors convert electric energy into mechanical energy in the form of ultrasonic vibrations to obtain a torque. By virtue of the advantages of silent operation and capabilities of providing a high torque by low-speed rotation and of permitting direct drive, consideration has been given to the application of ultrasonic motors to a wide variety of fields, for example, automotive functional parts, various kinds of robot, magnetism-optical disk memory, etc.
There have heretofore been proposed some types of ultrasonic motor, i.e., progressive wave type ultrasonic motor, standing wave type ultrasonic motor, and hybrid transducer type ultrasonic motor. In particular, the progressive wave type ultrasonic motor has been well examined technically because it provides stable motor performance for a long period of time.
As shown exemplarily in FIG. 3, a typical conventional progressive wave type ultrasonic motor comprises a vibrator (stator) 1 and a rotor 2. The stator 1 comprises a disk-shaped elastic member 11 made of a metallic material, for example, and provided with circumferentially spaced radial slits, and a ring-shaped piezoelectric ceramic 12 bonded underneath the elastic member 11. The rotor 2 comprises a disk-shaped elastic member 21 and a lining member 22 made of engineering plastics, for example, which is bonded to the elastic member 21. The rotor 2 is in press contact with the stator 1. By impressing a high frequency voltage matched with the resonance of vibration (resonance point) of the stator 1 on the piezoelectric ceramic 12, the stator 1 is caused to vibrate resonantly, thereby rotating the rotor 2 (Applied Physics 54 (1985) No. 6, pp. 589-590).
The piezoelectric ceramic 12 has an electrode structure such as that shown in FIG. 4. More specifically, the side of the piezoelectric ceramic 12 which is bonded to the elastic member 11 is provided with regions defined by the half-wavelength, a region (denoted by C in the figure) defined by the three-quarter wavelength, and a region (denoted by D) defined by the quarter-wavelength. On the other side of the elastic member 11, the regions defined by the half-wavelength are divided into two (denoted by A and B), as shown by the oblique lines, and provided in between the regions defined respectively by the quarter-wavelength and the three-quarter wavelength. In addition, the regions defined by the half-wavelength are subjected to polarization process to provide transverse piezoelectric effects which are alternately different in directivity.
When a high-frequency voltage is input to the regions A and B of the ring-shaped piezoelectric ceramic 12 with a phase difference of 90.degree. therebetween in a state where the stator 1 is placed in electrical resonance, a progressive wave traveling in one direction is excited on the surface of the elastic member 11. Accordingly, the motion of any one point on the surface of the elastic member 11 draws a locus such as that shown in FIG. 5. Since the vibrational amplitude shifts 90.degree. spatially and the phase shifts 90.degree. temporally, the bending vibration that is generated in the piezoelectric ceramic 12 generates an elliptical locus (a counterclockwise locus as viewed in FIG. 5) when the motion of the surface of the elastic member 11 is drawn on the basis of a virtual neutral axis. On the rotor 2, a frictional force acts in a direction (leftward as viewed in FIG. 5) counter to the direction (rightward as viewed in FIG. 5) of progress of the progressive wave, so that a torque is given to the rotor 2. Thus, the arrangement operates as a motor. It should be noted that the downward arrow shown in FIG. 5 represents the applied pressure.
The progressive wave type ultrasonic motor has the structural drawback that, since the rotor is pressed in direct contact with the upper side of the bending vibration generating portion of the elastic member, the vibration amplitude of the elastic member is undesirably held down to a small level. For this reason, it is necessary in order to obtain a high torque to raise the voltage applied to the piezoelectric ceramic to thereby increase the vibration compelling force produced in the elastic member. However, if the applied voltage is raised, the piezoelectric ceramic may be broken.
To overcome this drawback, an improved elastic member structure has been proposed wherein the elastic member 11 is separated into a portion which is brought into press contact with the rotor and a portion which is bonded to the piezoelectric ceramic in order to prevent the pressure applied to the rotor from directly affecting the piezoelectric ceramic, and two piezoelectric ceramics 121 and 122 are disposed over the entire circumference of the elastic member 11, as shown in FIGS. 6 to 10, thereby increasing the driving area, and thus improving the wavelength selectivity (e.g., Japanese Patent Laid-Open (KOKAI) Nos. 61-191277, 63-242185 and 63-262069, and U.S. Pat. No. 4,504,760). It should be noted that FIG. 10 shows vibratory pieces (driving force transmitting portions for transmitting vibrations to the rotor) 13 projecting radially from the elastic member 11 shown in FIGS. 8 and 9.
In any of these prior arts, however, no technical consideration has yet been given to the elastic member 11 as being an electric resonance system including electric and mechanical systems.
That is, the structure of the elastic member 11 is not designed by taking into consideration the best conditions for the interaction between the electric input system and the mechanical output system. To achieve the best conditions, it is particularly necessary to replace the elastic member 11 with an electric resonance system and electrically handle it as an admittance current in which energy periodically move between a condenser and a coil while being continuously consumed by a resistor. In the related arts, however, since the structure of the elastic member 11 is not regarded as an electric resonance system, an impedance gap is produced electrically and the resonance frequency increases mechanically, resulting in a very small vibrational amplitude. Accordingly, it is impossible to obtain a high torque when a motor is formed by use of the related art arrangement.